random fun fact we learned yesterday: the gold in ENIG finished boards is not there to be the thing that the solder wets and sticks to, it's only there to protect the nickel (which is a metal with very good wettability)
the gold layer in ENIG isn't as thin as it is to save cost, though that's part of it--it's intended to completely dissolve into the solder, exposing fresh nickel for the solder to wet
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@Felthry why do they plate it in gold and not just solder
@noiob Because gold prevents oxidation of the surface
if you're asking why not use a HASL finish (where the board is coated in solder), the answer is surface flatness--HASL can be bumpy enough to make some many-pinned packages fail to get all their pins attached. the problem is particularly pronounced with BGA packages that have more than a few dozen balls, which is why you should always go for ENIG over HASL for designs with large BGA parts in them
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@noiob I will note also that the amount of gold used in ENIG is incredibly tiny, the plating thickness is usually specified as 1 μ" which is a horrible unit, but which equates to somewhere around 100 gold atoms of thickness
so "gold is expensive" isn't really a major concern here, when you need so little of it to prevent the nickel from oxidizing
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@Felthry no, I get that, gold plating is cheap, see shitty HDMI cables
@noiob we've seen gold-plated *TOSLINK* cables
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@noiob you know, optical fiber. well known for being enhanced by gold plating
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@Felthry well, the plating is more for optics anyways
one reason for the nickel is as a barrier layer between the gold and copper, yes, because otherwise you get copper-gold intermetallics that can cause problems, but the other reason is to provide a very wettable surface under the gold for the solder to stick to
copper is even more wettable than nickel, but it dissolves into solder very quickly
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